The present invention generally relates to wireless communications, and particularly relates to selective colored interference suppression in a Rake-based communication receiver.
As a general proposition, a Code Division Multiple Access (CDMA) transmitter transmits K CDMA signals in parallel as a composite signal. An individual receiver recovers the ith one of these K signals by correlating the composite received signal with the ith one of the CDMA spreading codes used to generate the K CDMA signals. In this context, a “standard” RAKE receiver aligns each of one or more Rake “fingers” with the strongest multipath components of the received composite signal, despreads the selected multipath signals, and combines the finger signals to obtain a (Rake) combined signal for further processing.
The combining process involves the application of combining weights to the finger signals and a standard Rake receiver forms the combining weights as conjugates of the finger channel estimates, weighted according to multipath signal strength. Thus, the standard Rake receiver operates as a maximal ratio combiner but its performance is optimal only under white noise conditions.
In more detail, in the nominal case, the correlation process carried out in each finger cleanly recovers the desired ith signal and suppresses the remaining K−1 interfering signals. However, receiving multiple copies of the composite signal on different delay paths results in a loss of orthogonality between the K spreading codes and each finger signal thus includes some impairment arising from intra-cell interference, also called multiple access interference (MAI). These impairments exhibit cross-correlations because of the inherent correlations between the received multipath signals.
Generalized Rake (G-Rake) receivers offer improvements over the standard Rake receiver by incorporating knowledge of the impairment correlations into the combining weights. More particularly, G-Rake receivers measure or otherwise estimate the signal impairment cross-correlations for the different multipath delays and use that information to generate the combining weights in a manner that provides colored noise suppression as part of the Rake combining process.